A magnetic lens is often used for a charged particle beam optical system to converge a charged particle beam and at that time, the optical axis of a charged particle beam is arranged so that it is coincident with the principal axis of the magnetic lens. Besides, to enhance the resolution of observation and the precision of processing, the diameter of a charged particle beam is required to be reduced. Therefore, a specimen is often arranged in the vicinity of a center position of a magnetic field generated by the magnetic lens. Above all, in a transmission electron microscope, to acquire an image having little optical aberration and an image of high magnification, a specimen is often arranged in a strong magnetic field (1.5 to 2 T (tesla)) of a magnetic lens. In the meantime, to apply a magnetic field to a specimen and observe the magnetic property of the specimen, it is important to reduce an effect by a magnetic lens.
Therefore, as disclosed in a document (Sir P. Hirasch et al., “Electron Microscopy of Thin Crystals”, (1965) Robert E. Krieger Publishing Co., Inc.; Chapter 16.4), a method of turning off a magnetic field of a magnetic lens or a method of locating a specimen apart enough from a magnetic field of a magnetic lens is taken. Further, as disclosed in a paper (K. Shirota et al., J. Electron Microsc., Vol. 25 (1976) FIG. 1) and a paper (J. N. Chapman et al., IEEE Transactions on Magnetics, Vol. 60 (1994) 4479), a magnetic shielding lens in which a magnetic field is hardly applied to a specimen for locating the specimen off a magnetic circuit by a magnetic lens is developed.
In case a determined strong magnetic field is applied to a specimen in parallel with an optical axis, that is, in case a strong longitudinal magnetic field is applied to the specimen, the normal arrangement of a specimen and a method of using a magnetic lens may be used. To apply a weak magnetic field, a method of using a magnetic lens in an extremely weak excited state is taken as disclosed in a document (J. Chen et al., Optical Review Vol. 1 (1994) 304; p. 305 FIG. 3) and a method of using a magnetic field applying coil having an axis parallel to an optical axis is taken as disclosed in a document (T. Yoshida et al., J. Appl. Phys., Vol. 85 (1999) 4096; p. 4098 FIG. 3). A weak magnetic field parallel to an optical axis acts as a weak lens and the slight effect of rotation by Lorentz force upon a charged particle beam is produced, however, the quantity of displacement by rotation from the optical axis of a charged particle beam is small and can be fully corrected by a deflecting optical system with a normal charged particle beam optical system is provided beforehand.
In the meantime, in case a magnetic field is applied to a specimen perpendicularly to an optical axis, that is, in case a transverse magnetic field is applied to the specimen, Lorentz force that acts on a charged particle beam by the transverse magnetic field acts so that the charged particle beam is deflected from its optical axis. Therefore, normally, as disclosed in a paper (S. Hasegawa et al., Phys. Rev. B43 (1991) 7631; p. 7635 FIG. 5) or in Japanese published unexamined patent application No. Hei8-264146, in a system provided with a two-step beam deflector except a coil system for applying a magnetic field to a specimen, a charged particle beam is deflected again so that the charged particle beam travels along the optical axis of a lens system located at the back of the specimen. As a result, the deflection from the optical axis of the charged particle beam by Lorentz force that acts upon the charged particle beam is compensated and a transverse magnetic field is applied to the specimen.
FIG. 1 schematically shows an example in which a general three-step external magnetic field application-type beam deflector are arranged at an equal interval and symmetrical deflection is made. As shown in FIG. 1, a reference number 900 denotes an optical axis. A reference number 910 denotes the trajectory of a charged particle beam and the charged particle beam travels on the optical axis 900. Reference numbers 0701 and 0702 denote a pair of magnetic field applying coils and they generate a magnetic field B in a direction shown by an arrow in FIG. 1. For a magnetic field, constant field approximation (CFA) is used and a magnetic field region is dotted. The charged particle beam is deflected by the magnetic field. Reference numbers 0801 and 0802 denote a pair of magnetic field applying coils and they generate a magnetic field 2B in a direction shown by an arrow in FIG. 1. The charged particle beam is deflected in a direction reverse to the deflection by the magnetic field applying coils 0701 and 0702 by the magnetic field. Reference numbers 0901 and 0902 denote a pair of magnetic field applying coils and they generate the magnetic field B in the direction shown by an arrow in FIG. 1. The charged particle beam is deflected in a direction reverse to the deflection by the magnetic field applying coils 0801 and 0802 by the magnetic field and as a result, as shown in FIG. 1, is returned on the optical axis.
In FIG. 1, for convenience sake of construction, the charged particle beam is drawn so that it is deflected in the direction of the applied magnetic field, however, deflection actually caused by Lorentz force is made in a direction perpendicular to page space differently by 90 degrees from the direction of the applied magnetic field. The orientation depends upon charge which a particle has. In the case of an electron microscope, first, a charged particle beam is deflected in a direction toward the back of page space and next, is deflected in a direction toward the surface of the page space. At this time, assuming that for an angle deflected in order from the upside, a first angle is α, an angle of deflection has only to be adjusted so that a second angle is −2α (in a reverse direction) and a third angle is α and assuming that for a magnetic field (magnetic flux density) applied to each system, a magnetic field applied to a first system is B, a magnetic field applied to a second system is −2B and that applied to a third system is B. The two-step beam deflector used for deflection for returning is not necessarily required to be a magnetic field application type and may be also an electric field application type.
FIG. 2 shows another example showing a conventional type three-step external magnetic field application-type beam deflector. The example is different from the example shown in FIG. 1 in that to double the deflection angle of a second step (the center) because the magnetic field intensity of three steps is respectively B, −B, B and is equalized, the length on an optical axis of the corresponding magnetic field region in the deflection system is double, compared with that of upper and lower magnetic field regions, however, the examples are not substantially different in the effect of the deflection of a charged particle beam (assuming that the effect in a first step is α, the effect in a second step is −2α (in a reverse direction) and the effect in a third step is a). Also in FIG. 2, a displayed direction of deflection is different from an actual direction of deflection by 90 degrees as in FIG. 1.
A method of varying a component in a plane of a specimen of a magnetic field effectively applied to the specimen and a component in a normal direction of the plane by arranging the specimen in a magnetic field by a magnetic lens and changing the inclination of the specimen is also taken (for example, refer to a paper (T. Hirayama et al., Ultramicroscopy Vol. 54 (1994) 9; p. 11 FIG. 3). This method is convenient in that no deflection system for applying an external magnetic field to a specimen is used. However, as an angle between a charged particle beam and a specimen varies every condition for applying a magnetic field and a degree of the undersizing of an image by the inclination of the specimen also varies every time, the setting of a condition according to the object of observation is very difficult. Particularly, the method cannot be used for a case that the crystal orientation of a specimen and a direction in which an electron beam is incident are required to be strictly defined such as the observation of a crystal lattice using a high-resolution electron microscope.